To adjust the above composition, it is conventional to determine basic values for a fuel quantity as a function of the air quantity drawn in and the speed of the internal combustion engine and to correct these values by means of a superposed control.
Since the control system performs subsequent injections on the basis of prior measurements of an exhaust-gas sensor, delay times occur, for example, because of the transit time of the mixture between injection and measurement by the sensor.
When reaching a new operating point with basic values which are not optimal, a temporary incorrect adaptation of the mixture composition and therefore increased exhaust emissions occur. Basic values for a particular type of internal combustion engine, which have once been determined and stored, can lead to incorrect adaptations, for example, because of scatter between individual samples caused by the manufacturing process or because of drift phenomena caused by deterioration.
Continuous adaptation of the precontrol to these drift phenomena by means of a learning control method makes it possible to comply with exhaust regulations throughout the life of the internal combustion engine.
An example of a learning control method is known from U.S. Pat. No. 4,584,982.
When operating internal combustion engines with learning control methods, problems may occur in certain circumstances which do not occur in this way in internal combustion engines without learning control methods. It has been found that the correction based on the adaptation can initially reach values which appear implausibly high when the internal combustion engine is operated over short distances. In combination with the detection of a fault based on an implausible adaptation value, this can lead to an unnecessary fault signal. Since the implausibly high values act in the direction of mixture leaning, it is moreover impossible to rule out difficulties in subsequent starts because of excessive leaning of the mixture.